The present invention relates to precision laser scanning for machining or cleaning a work piece. More particularly, the present invention relates to a method and apparatus for high precision laser scanning for micro machining, sub-micro machining, or cleaning using an ultrashort pulsed laser beam and an acoustooptical deflection device. The present invention is useful for high-precision machining of a variety of structures, including photolithographic masks. The invention is not intended to be limited to the above-noted uses, however.
Laser machining has significant applications in the automobile, aerospace and electronics industries for cutting, drilling and milling. Salient features of laser machining include the ability to make small and unique structures and the ability to process hard-to-work materials such as ceramics, glasses and composite materials. High-power CO2 and YAG lasers are conventionally used for laser machining. However, conventional laser machining suffers from problems including rough machining kerf, existence of a recast layer and large heat-affected zone, and restriction to large feature sizes. Hence, conventional laser machining is not suitable for precision micromachining.
In contrast, in machining based upon ultrashort pulsed lasers, the mechanism of material removal is different from that of conventional lasers. Moreover, the heat affected zone is negligibly small, and the melt zone is virtually absent in ultrashort pulsed laser machining. Hence, laser machining using ultrashort laser pulses is suitable for precision micromachining.
One aspect of ultrashort pulsed laser machining (also referred to as ablation) of interest is the effect of beam polarization. PCT Patent Publication No. WO 99/55487 entitled xe2x80x9cMethod and apparatus for improving the quality and efficiency of ultrashort-pulse laser machiningxe2x80x9d by Hoang et al. discloses controlling the polarization of an ultrashort pulsed laser beam in a machining process. However, the physical mechanism underlying the relationship between polarization and its effect on machining has not been explained.
In addition, in conventional ultrashort pulsed laser machining, features are generated by scanning the ultrashort pulsed laser beam using mechanical scanning systems, such as galvano mirrors. A main disadvantage of these systems is that they are prone to vibration, which can adversely affect the positional accuracy of the scanned beam. This effect can be very detrimental in attempts to machine features of submicron size. Also, the use of mechanical scanning systems limits scanning speed and spatial resolution of the system.
Applicants have recognized that using acoustooptic devices instead of mechanical systems for beam scanning can avoid the above-noted vibration problem. Applicants have also recognized that the use of acoustooptic devices can lead to noticeable dispersion of the ultrashort pulsed laser beam. It would be desirable to have an ultrashort pulsed laser scanning system where dispersion from acoustooptic devices is compensated.
It should be emphasized that the terms xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
In one aspect of the invention, a precision laser-scanning apparatus is provided. The precision laser-scanning apparatus comprises a laser source that emits a pulsed laser beam, a dispersion compensation scanner that scans the pulsed laser beam, and a focusing unit that focuses the pulsed laser beam from the dispersion compensation scanner on a work piece. The dispersion compensation scanner comprises a first scanning device that scans the pulsed laser beam in a first direction and that causes dispersion of the pulsed laser beam. The dispersion compensation scanner further comprises a first dispersion compensation device that compensates for the dispersion caused by the first scanning device.
The pulsed laser beam may have a wavelength in the range of approximately 100 nm to approximately 1500 nm, a pulse width in the range of approximately 1 microsecond to approximately 1 femtosecond, and a pulse repetition rate in the range of approximately 10 hertz to approximately 80 megahertz. In addition, the dispersion compensation scanner may have a scanning resolution in the range of approximately 1 nanometers to approximately 100 micrometer depending on the scanning range. Further, the dispersion compensation scanner may have a scanning random access time in the range of approximately 0.01 microseconds to 100 microseconds depending on the rise time of the acoustic crystal.
In another aspect of the invention, the pulsed laser beam of the precision laser-scanning apparatus may have a pulse energy and an intensity sufficient to machine a surface of work piece. The apparatus may have a spatial machining resolution in the range of approximately 0.05 micron to approximately 100 micron. Further, the apparatus may have a spatial machining resolution of approximately one-twentieth of a cross-sectional diameter of the pulsed laser beam in a focused state at the surface of the work piece. Moreover, the apparatus may further comprise a polarization converter that provides a polarization state to the pulsed laser beam. The polarization converter may be selected from the group consisting of a quarter-wave plate, a half-wave plate, and a depolarizer. In addition, the polarization converter may be selected to provide the polarization state depending upon a desired shape of a machined feature to be generated on a surface of the work piece or depending upon an ablation threshold of the work piece. Further, the polarization state may be selected to be one of a linear polarization state for machining an elliptical feature, a circular polarization state for machining a circular feature, and a random polarization state for machining a circular feature.
In another aspect of the invention, the apparatus may further comprise a beam filter that spatially filters the pulsed laser beam and provides the pulsed laser beam with a desired cross-sectional size. The beam filter may comprise a pin hole with a diameter approximately equal to or greater than a desired spot size of the pulsed laser beam when focused onto the work piece, a first focusing lens that focuses the pulsed laser beam onto the pin hole, a second focusing lens that collimates the pulsed laser beam emanating from the pin hole, and a diaphragm that blocks an outer portion of the pulsed laser beam emanating from the second focusing lens.
In another aspect of the invention the dispersion compensation scanner may further comprise a second scanning device that scans the pulsed laser beam in a second direction different (or perpendicular) from the first direction and that causes dispersion of the pulsed laser beam, wherein the first dispersion compensation device compensates for dispersion caused by both the first scanning device and the second scanning device. The first and second scanning devices may be acoustooptic devices, and the first dispersion compensation device may be a diffraction grating having a line spacing suitable for compensating the dispersion caused by the first and second scanning devices. Alternatively, the first dispersion compensation device may be an acoustooptic device. The first and second scanning devices are acoustooptic deflectors, and the first dispersion compensation device is an acoustooptic modulator that further provides modulation of the pulsed laser beam wherein the beam is selectively transmitted or blocked with a precision ranging from 1 nanosecond to 20 microseconds. The first and second scanning devices may be oriented such that the first direction is perpendicular to the second direction, and the first dispersion compensation device may be oriented at an angle relative to the first scanning device such that the first dispersion compensation device produces a negative dispersion that counters the resultant dispersion caused by the first and second scanning devices. In addition, the dispersion compensation provided by the first dispersion compensation device may result in a dispersion error below approximately 30% over the entire scan range. The first compensation device, the first scanning device and the second scanning device may be made of the same material and may have the same acoustic velocity, and the center acoustic frequency of the first compensation device may chosen such that dispersion compensation is optimized for the entire scanning range of the apparatus.
In another aspect of the invention, the dispersion compensation scanner may further comprise a second scanning device that scans the pulsed laser beam in a second direction different from the first direction and that causes dispersion of the pulsed laser beam and may further comprise a second dispersion compensation device that compensates for dispersion caused by the second scanning device. The first and second scanning devices may be acoustooptic devices, and the first and second dispersion compensation device may be diffraction gratings having a line spacing suitable for compensating the dispersion caused by the first and second scanning devices. Alternatively, the first and second dispersion compensation devices may be acoustooptic devices. The first and second scanning devices may be acoustooptic deflectors, and the first and second dispersion compensation devices may be acoustooptic modulators that further provide modulation of the pulsed laser beam wherein the pulsed laser beam is selectively transmitted or blocked. In addition, the first and second scanning devices may be oriented such that the first direction is perpendicular to the second direction, and the first and second dispersion compensation devices may be oriented such that a direction of an acoustic wave in the first dispersion compensation device is perpendicular to a direction of an acoustic wave in the second dispersion compensation device, the first and second dispersion compensation devices producing a negative dispersion that counters the resultant dispersion caused by the first and second scanning devices. The dispersion compensation provided by the first and second dispersion compensation devices may result in a dispersion error below approximately 30% over the entire scan range. The first and second compensation devices and the first and second scanning devices may be made of the same material and have the same acoustic velocity, and a center acoustic frequency of the first and second dispersion compensation devices may be chosen such that dispersion compensation is optimized for the entire scanning range of the apparatus.
In another embodiment of the invention, a method of scanning a pulsed laser beam on a work piece for machining or cleaning the work piece is provided. The method comprises emitting a pulsed laser beam from a laser source, focusing the pulsed laser beam on the work piece, scanning the pulsed laser beam on the work piece using a first scanning device that scans the pulsed laser beam in a first direction and that causes dispersion of the pulsed laser beam, and compensating for the dispersion caused by the first scanning device using a first dispersion compensation device.
The method may further comprise machining a surface of the work piece by ablating material from the surface. In addition, the method may further comprise converting a polarization of the pulsed laser beam to a predetermined polarization state prior to scanning the pulsed laser beam on the work piece. The method may further comprise selecting the predetermined polarization state depending upon a desired shape of a machined feature to be generated on the surface of the work piece or depending upon an ablation threshold of the work piece. The method may further comprise spatially filtering the pulsed laser beam to provide the pulsed laser beam with a desired cross-sectional size.
In another aspect of the invention the method may further comprise scanning the pulsed laser beam in a second direction different from the first direction using a second scanning device that causes dispersion of the pulsed laser beam, and compensating for dispersion caused by both the first scanning device and the second scanning device using the first dispersion compensation device. The method may further comprise modulating the pulsed laser beam using the first dispersion compensation device, the first dispersion compensation device being an acoustooptic modulator. The method may further comprise scanning the pulsed laser beam in a second direction different from the first direction using a second scanning device that causes dispersion of the pulsed laser beam, and compensating for dispersion caused by the second scanning device using a second dispersion compensation device. The method may further comprise modulating the pulsed laser beam using the first and second dispersion compensation devices, the first and second dispersion compensation devices being acoustooptic modulators.